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Batteries and maintenance
14.15 Theory and Facts on Car
Batteries
In winter and cold conditions, the battery capacity
is considerably reduced.
Battery capacity can be measured by measuring
the density of the battery acid and the battery's
rest potential. A fully charged battery has a rest
potential of about 12.7 volts. The surest result is
with a density meter. Fully charged battery acid
has a density of 1.28 g/cm³ at + 25 °C. This density
is reduced together with the voltage as the battery
loses charge.
As a rule of thumb, you can say that a fully
charged battery at -18 °C supplies about 40-
50 %current and at -30°C only 25-30 %. A cold
battery cannot receive current properly either. If
the density drops below 1.28 g/cm³, lead sulfate
is formed on the battery cells and if the situation is
allows to continue, the cells will become clogged.
The sulfate shortens the service life of the battery
and impairs the ability of the battery to supply and
receive current. It is therefore extremely important
that the battery is kept fully charged and to be fully
charged it must first become warm.
14.16 Lead Sulfate, What is It?
Sulfuric acid (H
SO
) diluted with water is used
2
4
in lead batteries. Blades of lead oxide (PbO
used as positive electrodes and discs of lead (Pb)
as negative. When current is lead into the battery,
for example from the battery charger, the lead
oxide and the lead start to react with the sulfuric
acid to form lead sulfate on the electrodes.
The sulfuric acid content, that acts as an
electrolyte, also changes so that its strength
(density) increases when the battery is charged
and is reduced when the battery loses charge.
When the battery is charged, lead is freed from the
lead sulfate on the negative electrode again and
the lead sulfate on the positive electrode turns to
lead oxide.
In winter, if the battery is cold and cannot be fully
charged, too much lead sulfate is formed and clogs
the cells. The lead sulfate cannot be removed from
the clogged cells, no matter how much the battery
is charged. The only solution is to buy a new
battery.
A lead battery should never be discharged more
than 75% to avoid damage. This means the
voltage has dropped to 12.4 volts and there is
an acute need of charging. This situation usually
already arises at 0°C even when the battery
142
English –
has been fully charged in warm conditions. Cold
therefore considerably reduces the service life
of a battery if current is take out of it in the cold.
However, it is fully possible to store a fully charged
battery in cool or cold conditions as long as it is
fully charged, self-discharging is reduced in the
cold.
The recommended operating temperature of a
lead battery is 10-30 °C. The warmest and coldest
temperatures reduce the service life of the battery
considerably. The highest permitted temperature
is 55 °C. Battery operating temperatures of 45 °C
should be avoided.
The best way to avoid sulfating of the battery is to
maintain as high a level of charge as possible the
whole time.
2
) are
2
1. Terminals
2. Positive plates
3. Negative plates
4. Insulation
5. Battery casing
14.17 Charge Voltages for Three Series
Connected Lead-Acid Batteries
Maintenance charge = 39.6V
Charge voltage = 43.2V
1
3
4
5
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Troubleshooting
